1. Field of the Invention
The present invention relates to a method of applying a liquid coating composition onto a moving support by curtain coating, and more particularly, to a method of applying a variety of liquid coating compositions onto the support without causing non uniformity in layer thickness adjacent the edges.
2. Description of the Prior Art
While curtain coating has been developed for many years in the painting and packaging fields, coating operations exhibiting greater accuracy and higher productivity are in demand. To meet this end, U.S. Pat. No. 3,508,947, 3,232,743, Japanese Pat. Application (OPI) Nos. 74761/77 and 74762/77 (the term "OPI" as used herein refers to a "published unexamined Japanese Pat. application") have proposed the application of curtain coating in the manufacture of photographic materials, pressure-sensitive copying paper and heat-sensitive recording paper.
Curtain coating fundamentally consists of the rapid horizontal movement of the support (the flat article to be coated) through a free falling curtain supported by two edge guides at both ends in a transverse direction. But if the entire width of the curtain is applied to the support using these edge guides, the coating applied at the edges of the support would be thicker than that applied to the major portion of the support. This presents many problems in the industrial application of the curtain coating process. First, this would complicate the drying problem unless the edges of the support were subsequently trimmed off. Or, if the drying section does not have sufficient capability to dry the thick coat, it remains only partially dry and the wet coating solution fouls the surface of the transport rollers in subsequent steps, or both edges of the support stick to the winding rolls so strongly that the support often breaks when it is unwound in the slitting step. It is therefore necessary to prevent this phenomenon by giving the drying apparatus the additional capacility to dry the thick coat.
Secondly, the thick coat is only about a few milimeters wide and is not included in the final product but is simply discarded. It is certainly uneconomical to spend extra money in drying such undesired portions.
Thirdly, even if the thick coat is appropriately dried, the web when wound about the winding roll many times forms a ridge on both ends due to the accumulation of the thick coat, and if the formation of such ridge is excessive, a support of thin plastic sheet deforms when it is wound up, and a paper support breaks apart on that ridge to make the winding of a sufficiently long web difficult. In either case, the production efficiency is greatly reduced.
It is known that a curtain is generally made wider than the support to be coated to ensure that the coating applied to the support is of uniform thickness right to its edges. When a single component layer is formed on the support by this technique, the liquid coating composition in that part of the curtain which extends beyond the side edges of the support and which simply falls down without being applied to the support is recovered and recycled for further use.
Methods of coating superposed layers on the support are described in U.S. Pat No. 4,019,906, U.S. Pat. No. 4,135,477, and U.S. Pat. No. 4,233,346,wherein only some of the plurality of layers are, and in the extreme case only one layer is, in contact with the curtain guides. The width of the remaining layers is smaller than the width determined by the curtain guides, and preferably smaller than the width of the web. All of the free falling curtain composed of a plurality of layers is coated onto the support wherein the portion of said one, broader layer that overflows the edges of the web is collected and recirculated. These methods are known to be capable of overcoming the problems related with the edge regions of the curtain.
The above described methods depend on the formation of a curtain wider than the support. But even if a uniform thickness is maintained right to its edges, those portions of the support often have inherently poor properties and are not usually included in the final product, so the liquid coating composition applied to those portions is simply wasted. In addition, the coating machine is designed so that the liquid coating composition is applied to the entire surface of the support, so it sometimes occurs that the liquid coating composition is undersirably applied to the side edges and fouls the rolls or belts that transport the support. If a paper substrate is used as in the manufacture of pressure-sensitive copying paper or heat sensitive recording paper, the sides of the paper coated with the liquid coating composition curl downward to a great extent, and as a result, the curled side bands contact the transport rolls and foul them with the liquid coating composition before the web enters the drying zone.
Even in the drying step, the curled side bands contact and foul the canvas belt or rolls before the web becomes completely dry. In a "floater dryer" as described in Japanese Patent Application (OPI) No. 25066/79 which achieves both web transport and drying without contacting any transport means, the curled side bands "shudder" and frequently foul the nozzles of the floater dryer.
If the fouling of the equipment is continued, the undesired buildup of the liquid coating composition may cause the paper substrate to break or to be dislodged from the equipment and undesirably deposited on a freshly coated surface to damage it.